A hazardous exhaust system shall be required wherever operations involving the handling or processing of hazardous materials, in the absence of such exhaust systems and under normal operating conditions, have the potential to create one of the following conditions:
- A flammable vapor, gas, fume, mist, or dust is present in concentrations EXCEEDING 25% of the lower flammability limit (LEL) of the substance for the EXPECTED ROOM TEMPERATURE
- A vapor, gas, fume, mist, or dust with an NFPA health-hazard rating of 4 is present in any concentration
- A vapor, gas, fume, mist, or dust with an NFPA health-hazard rating of 1, 2, or 3 is present in concentrations EXCEEDING 1% of the median lethal concentration of the substance for acute inhalation toxicity
Exception: Laboratories, as defined in Section 510.1, except where the concentrations listed in Item 1 are exceeded or a vapor, gas, fume, mist, or dust with a health hazard rating of 1, 2, 3, or 4 is present in concentrations exceeding 1 percent of the median lethal concentration of the substance for acute inhalation toxicity.
This section specifies where the installation of an effective hazardous exhaust system is required based on the stated conditions. This section contains the criteria to determine whether a given operation requires a hazardous exhaust system. If, in the absence of an exhaust system, the normal operation would exceed the threshold concentrations indicated, a hazardous exhaust system is required. The code does not indicate how to determine if any of the criteria are triggered. It may be necessary to consult an expert in the field.
The hazardous exhaust system is intended to keep the threshold levels from being reached, either in the room containing the point source (for all types of hazardous materials) or in the exhaust duct system (for flammability/explosion hazards). The CRITERIA ARE BASED ON NORMAL OPERATING CONDITIONS, NOT ON ACCIDENT CONDITIONS. It is important to note and emphasize that the threshold concentrations are without the exhaust system operating. Section 510.3 requires the exhaust system to operate below threshold concentrations; therefore, the threshold limits are determined without the exhaust system operating.
Item 1 requires a hazardous exhaust system where FLAMMABLE VAPOR, GAS, FUME, MIST, OR DUST REACHES CONCENTRATIONS THAT EXCEED 25% OF THE LOWER FLAMMABILITY LIMIT (LFL) of the substance for the range of anticipated space temperature(s). The limits of flammability (high and low) are the extreme concentration limits of a substance. The LFL of a substance is the lowest concentration by volume of flammable vapor, gas, fume, mist, or dust mixed with air (or any oxidant) that will ignite and burn with a flame. For example, hydrogen-air mixtures will propagate flames in concentrations between 4 and 74% by volume of hydrogen at 70°F (21°C) and at atmospheric
pressure. The smaller value is the lower (lean) limit, and the larger value is the upper (rich) limit of flammability.
For purposes of this example, anything greater than a 1% concentration by volume of hydrogen in the air would exceed 25% of hydrogen’s LFL at 70°F (21°C) and atmospheric pressure. Keep in mind that when the mixture temperature is increased, the flammability range WIDENS; when the temperature is decreased, the range NARROWS. The term “lower flammability limit” describes the minimum concentration by volume of vapor to air that will propagate a flame in the presence of an ignition
source. The upper flammability limit (UFL) is the maximum vapor-to-air concentration that will propagate a flame. If a vapor-to-air mixture is below the LFL, it is described as being “too lean” to burn, and if it is above the UFL, it is “too rich” to burn. When the vapor-to-air ratio is somewhere between the LFL and the UFL, fires and explosions can occur upon introduction of an ignition source. The mixture is then said to be within its flammable or explosive range. When the mixture is in the range between the LFL and UFL [synonymous with LEL (lower explosive limit) and UEL (upper explosive limit)], the ignition is more intense and violent than if the mixture were closer to either the upper or lower limits.
Item 2 requires a hazardous exhaust system where the general HEALTH HAZARD POSED BY A SINGLE EXPOSURE TO A VAPOR, GAS, FUME, MIST, OR DUST IS CLASSIFIED WITH A NFPA 704 HEALTH-HAZARD RATING OF 4. Health-hazard ratings are quantified on an intensifying numerical scale from zero to 4, with zero posing no health hazard beyond that of ordinary combustible material and 4 being too dangerous to health to expose even the best-trained personnel. For a material with a health hazard rating of 4, a few whiffs of the vapor, gas, fume, mist, or dust could cause death, or the vapor or liquid could be fatal upon penetrating the full protective clothing of a firefighter. Exposure may vary from a few seconds up to an hour. However, the intent here is to consider hazardous vapors, gases, fumes, mists, or dust that may arise out of an inherent property of the exposed material in the presence or absence of a catalyst.
Item 3 requires a hazardous exhaust system where the CONCENTRATION OF A PARTICULAR VAPOR, GAS, FUME, MIST, OR DUST WITH A HEALTH-HAZARD RATING OF 1, 2, OR 3 IS PRESENT IN EXCESS OF 1% OF THE MEDIAN LETHAL CONCENTRATION OF THE SUBSTANCE FOR ACUTE INHALATION TOXICITY. Health-hazard rankings are based primarily on criteria detailed in the United Nations (UN) publication, Recommendations on the Transport of Dangerous Goods. The UN criterion for inhalation toxicity is based on the LC50 (lower concentration limit fatal to 50 percent of the test population) and saturated vapor concentration of the material. Furthermore, in addition to inhalation toxicity, the UN has established criteria for oral (exposure by ingestion) and dermal (exposure by skin) toxicity, as well as corrosivity; all of which have been considered in developing NFPA 704.
The exception for laboratories is to recognize the occasional presence of Class 4 materials in laboratories in such small concentrations to preclude the need for a hazardous exhaust system. This exception does not change the threshold limits for Items 1 and 3. If these limits are exceeded, then a hazardous exhaust system for the laboratory is required. This exception has the effect of making the requirements of Item 2 less stringent. Instead of having the threshold for a vapor, gas, fume, mist, or dust classified as a health-hazard rating of 4 at any concentration, it will now be treated as hazard ratings 1, 2, or 3 in Item 3. In effect, the exception simply exempts labs that have some chemicals with a health-hazard rating of 4 if the concentrations of such chemicals do not exceed 1% of the median lethal concentration. Minute quantities used in laboratories do not necessarily drive the requirement for a hazardous exhaust system. This exception addresses the need for a hazardous exhaust system when hazardous quantities of Class 1, 2, 3 or 4 materials are in use in any lab environment at concentrations exceeding the prescribed limit.